Mouse fibroblasts transformed with the thymidine kinase gene derived from herpes simplex virus acquire the metabolic properties which allow them to grow in selective media if the thymidine kinase gene is integrated. Co-transformation of these cells with the thymidine kinase gene in addition to a sample of DNA which includes a globin gene may result in stable integration of both genes allowing the expression of the globin genes to be studied. We have obtained eleven stable cell lines by co-transformation of mouse fibroblasts with the thymidine kinase gene and a DNA segment which includes the human delta and beta globin genes. If the human globin genes are expressed in these transformed cells, we anticipate using this system to characterize the mechanism of their regulation, and using analogous transformation experiments, to explore the molecular defects that affect the beta globin in patients with beta thalassemia.